1. Field of the Invention
The present invention relates to a process for preparing N,N'-disubstituted urea and more particularly, to an improved process for preparing N,N'-disubstituted urea derivatives of the following formula (I) ##STR2## wherein Ar represents an unsubstituted aromatic radical or an aromatic radical substituted with a halogen atom, an alkyl group, or an alkoxy group, which comprises reacting an aromatic mono-nitro compound, an aromatic primary amine, and synthesis gas in the presence of a catalyst consisting essentially of a divalent palladium compound as a main catalyst component and an ammonium or a phosphonium salt containing halogen atom as a co-catalyst component, and a non-polar solvent.
2. Description of the Prior Art
The conventional N,N'-disubstituted urea is an important intermediate for the production of carbamates which are raw materials for agrochemicals. The conventional methods for preparing N,N'-disubstituted urea have heretofore been developed by reaction of amines with carbon monoxide in the presence of non-metal catalysts such as tertiary aliphatic amine (J. Org. Chem., 26, 3309, 1961), selenium (J. Am. Chem. Soc., 93, 6344, 1971), and metal catalysts such as cobalt carbonyl (Can. J. Chem., 40, 1718, 1962), and silver acetate (J. Org. Chem., 37, 2670, 1972).
The disclosure in J. Org. Chem., 26, 3309, 1961 relates to a method to synthesize ureas using tertiary aliphatic amine as a catalyst, but the yield of urea is 86.2% only when an excess amount of sulfur is reacted with amine and carbon monoxide. The use of sulfur produces unwanted by-product hydrogen sulfide and its treatment imposes additional cost to the process economy.
A method of the preparation of ureas as by synthesizing ureas from aliphatic amines, carbon monoxide, and oxygen using selenium as a catalyst is disclosed in J. Am. Chem. Soc., 93, 6344, 1971. The selenium catalyst has a well-known toxicity problem. A continuous flow of oxygen is also required in the method to precipitate the selenium catalyst, which is not only dangerous due to the explosion but also expensive. Those methods using catalysts such as sulfur, selenium, etc., have high yield and selectivity. However, it is very difficult to separate and recover those catalysts. That is, unless the catalysts can be separated for reuse, the catalyst loss generally tends to make the expense of using the process prohibitive for economic purpose.
U.S. Pat. No. 4,052,454 also discloses a process for the production of ureas by synthesizing disubstituted urea with a selenium or sulfur catalyst. The maximum yield of the disclosure using the selenium catalyst is limited to 67.3%. For the sulfur compound, which is the catalyst actually claimed in the disclosure, the urea yield given in the example is merely 3.4-7.7%, and it is clearly impractical. The disclosure suggests to use more amine in moles than nitro compound. However, this method is limited to the preparation of unsymmetric urea. For the preparation of symmetric urea, the use of water is proposed.
Other methods for preparing N,N'-disubstituted urea in the presence of metal catalysts except platinum group catalysts are not practical since the yield and selectivity of the reaction is quite low.
The processes using platinum group catalysts are disclosed in European Patent Publication No. 319,111, Japanese Patent Publication No. 53-41,123, Japanese Patent Laid-Open Publication Nos. 58-144,363 and 62-59,253, and J. Org. Chem. Vol. 40 (19), 2819, 1975.
Among such disclosures, European Patent Publication No. 319,111 uses a salt of Cu, Fe, Mn, V, Cr, Zn, Sn, U, or Ce in addition to a palladium compound. It is well known that these salts promote the reaction called the Wacker-type chemistry, which involves a redox cycle between Pd(II) and Pd(O) with a metal salt serving as a reoxidant of Pd(O) to active Pd(II) species. Hence, the presence of a reoxidant is a necessity in this type of reaction. Otherwise, the catalyst is separated as a zero-valent metal, and the efficiency of the reaction decreases. When an excess amount of amine is used, unsymmetric urea is mainly obtained by this method. The best example given in this disclosure is 73% yield of 1,1-dimethyl-3(4-chlorobenzene)-urea for 20 hours of reaction time.
Japanese Patent Publication No. 53-41,123 and Japanese Patent Laid-Open Publication No. 58-144,365 relate to a process for preparing N,N'-disubstituted urea by reaction of amines with carbon monoxide under an elevated temperature and a high pressure. In such prior art methods, it is not only difficult to control the partial pressure of two kinds of gases involved, i.e. carbon monoxide and oxygen, but also there is a danger of explosion due to the oxygen.
A method disclosed in J. Org. Chem. Vol. 40 (19), 2819, 1975, describes a method of synthesizing N,N'-diaryl urea at 1 atm, 90.degree. C. with the initial amine/nitro compound molar ratio less than 1. In this disclosure, tri-n-butyl amine is used up to 50% together with solvent to maintain the catalyst activity. Otherwise, the activity of the catalyst is suddenly decreased during the reaction. Since a small amount of aromatic primary amine is employed, the decomposition of catalyst is not effectively inhibited. Hence, the catalyst cannot be reused and recycled, and the activity of the catalyst is reduced to 50% of the initial activity after one cycle of the reaction. Although it is ideal for this reaction to dissolve the palladium catalyst completely in the aromatic primary amine, it is actually very difficult to achieve due to the low amine concentration in the reaction mixture. Furthermore, the reaction pressure is too low to conduct an efficient carbonylation reaction. As a result, the yield of the aromatic urea according to the method is as low as 64%.
The process disclosed in Japanese Patent Laid-Open Publication No. 62-59,253 gives relatively high yield and selectivity. However, it requires expensive catalysts such as rhodium and ruthenium compounds. Furthermore, the appearance of the resulting N,N'-disubstituted urea is not neat, and the catalysts are unstable at a high temperature and decomposed around the reaction temperature.
All the above methods use pure carbon monoxide as a raw material. Since carbon monoxide is usually separated from synthesis gas by cryogenic distillation or solvent absorption, additional investments for the purification of carbon monoxide are required. On the other hand, the synthesis gas itself, consisting of mainly carbon monoxide and hydrogen in various proportions, can be manufactured cheaply by the methods such as coal gasification, partial oxidation or steam cracking of natural gas and oil, etc. When the synthesis gas is used instead of pure carbon monoxide as a raw material for urea synthesis, it will be superior in the economy of the raw material than any other process disclosed so far. Furthermore, when the hydrogen in synthesis gas reacts with a mono-nitro compound producing a corresponding amine during the synthesis reaction of urea such as formula (I), it can reduce the consumption of the amine which is more expensive than the corresponding mono-nitro compound, and the process economy will be greatly improved.
Inorg. Chem., Vol. 9,342, 1970 disclosed a method of N,N'-disubstituted urea synthesis by reacting nitrobenzene with synthesis gas. The method requires high pressure of carbon monoxide and the yield of urea is as low as 54%.
In order to avoid such problems, the present inventors are also prosecuting another U.S. patent application Ser. No. 07/606,721, filed on Oct. 31, 1990, now allowed, which is fully incorporated herein by reference and discloses a process for the preparation of N,N'-disubstituted urea derivatives comprising reacting an aromatic mono-nitro compound, aromatic primary amines, and carbon monoxide in the presence of catalysts. However, it is difficult to separate the carbon monoxide and require a large amount of the aromatic primary amines.